Turbomachines such as heavy-duty gas turbines and jet engines operate in extreme environments, exposing the turbine components, especially those in the turbine hot section, to high operating temperatures and stresses. In order to maintain the mechanical integrity of hot section components, one of two approaches is conventionally used. In one approach, cooling air is used to reduce the part's effective temperature. In a second approach, the component size is increased to reduce the stresses. However, these approaches can reduce the efficiency of the turbine and increase the cost.
In order for the turbine components to endure the high temperatures and stresses in the hot section, they are manufactured from a material capable of withstanding these severe conditions. In certain applications, superalloys have been used in these demanding applications, because superalloys maintain their strength at up to 90% of their melting temperature and have excellent environmental resistance. In certain other applications, nickel-based superalloys, in particular, have been used extensively throughout gas turbine engines, e.g., in turbine blade, nozzle, wheel, spacer, disk, spool, blisk, and shroud applications. In some lower temperature and stress applications, steels may be used for manufacturing turbine components. However, conventional steels cannot currently be used in high temperature and high stress applications because they do not meet the necessary mechanical property requirements.
In particular, for heavy duty land based gas turbine wheels operating at higher temperatures, for example at operating temperatures greater than approximately 1000 degrees Fahrenheit, conventional steels do not have the necessary mechanical properties. As a result, nickel-based superalloys strengthened with a gamma double prime phase are conventionally used in such applications. However, at such higher temperatures, the hold time crack growth resistance (HTFCG) of most gamma double prime strengthened nickel-based superalloys may not meet design requirements.
It should be noted that for heavy duty gas turbine wheels, critical mechanical properties change from the bore to the rim of the wheel. For example, the bore is limited by burst strength, and hence would require a higher ultimate tensile strength. The rim is limited by a material's creep life and its resistance to HTFCG. Many gamma double prime nickel-based superalloys cannot meet the HTFCG resistance required at elevated temperatures.
Accordingly, it is desirable to have an enhanced alloy article, that is capable of maintaining its mechanical integrity over a range of conditions ranging from higher stress/lower temperature to higher temperature/lower stress.